1. Field of the Invention
The present invention relates to a method and apparatus for recording information on an optical recording medium, and in particular, to a method and apparatus for irradiating the optical recording medium with a laser beam using pulse control during recording, wherein the method and apparatus take into account the conditions of the medium during reproduction.
2. Description of the Related Art
Various standards of optical recording media such as CD-R/RWs and DVD-R/RWs, which allow the user to record information, have been widely used. Meanwhile, the demand for large storage capacity in these types of optical recording media is increasing year by year. In order to meet this demand, new standards such as Blu-ray discs (BDs) have been proposed. An optical disc apparatus under the Blu-ray standard uses a data recording/reproduction laser beam with a narrow beam spot diameter. Specifically, the apparatus uses a laser beam with a shorter wavelength λ and a higher numerical aperture (NA) objective lens for condensing the laser beam. As a result, a Blu-ray disc is capable of recording about 25 GB data on its information recording layer.
In general, rewritable optical recording media, which allow rewriting data, use a recording film made of a phase change material. Specifically, by heating the recording film by irradiation with a laser beam and appropriately controlling the cooling rate thereafter, an amorphous region and a crystal region are selectively formed in the film, and the difference in reflectivity between these regions achieves recording information. To do so, the laser beam is required to be set under various conditions such as a write power level (Pw) with a high energy, an erase power level (Pe) with a middle energy, and a bias power level (Pb) with a low energy. Recording information on the optical recording medium is performed by pulse irradiation of the laser beam with any selected one of the three power levels.
Irradiating the optical recording medium alternately with a write pulse with the write power level and a bias pulse with the bias power level forms a mark thereon. The recording film is irradiated with the write pulse, so that the irradiated region is heated to its melting point or more. When the same region is then irradiated with the bias pulse, the area is rapidly cooled and thus is turned to an amorphous recording mark. Therefore, if the set of the write pulse and the bias pulse is continuously applied, it is possible to form a long recording mark.
Irradiating the optical recording medium with an erase pulse with the erase power level erases a mark having been recorded on the optical recording medium. The recording film is irradiated with the erase pulse, so that the irradiated region is heated to its crystallization temperature or more. The whole irradiated region, including the amorphous region (mark), is then crystallized by natural cooling, so that the recording mark is erased.
Accordingly, recording information is performed in such a manner that a region where a mark is to be formed is continuously irradiated alternately with the write pulse and the bias pulse according to the length of the mark, and a region where a space is to be formed is irradiated with the erase pulse according to the length of the space. This power modulation is called a write strategy.
Increasing the recording density as well as the write speed causes an edge shift on the recording mark. For example, when a long recording mark such as 6T is formed by a plurality of write pulses, too high write speed makes it hard to ensure sufficient cooling time, which corresponds to the width of the bias pulse between the write pulses. This poor cooling causes a part of the mark to recrystallize, and thus may degrade the recording quality. Therefore, in order to increase the recording accuracy, accurate control of the laser pulse is required, and there are various approaches therefore.
For example, Japanese Patent Application Laid-Open No. 2005-71516 discloses that, when writing each mark, the width of a bias pulse which is inserted into the end of the mark is varied depending on the length of the mark so as to reduce the jitter of a production signal. Moreover, when writing each mark, a bias pulse is inserted additionally into the top of the mark so as to delay the rising of the first write pulse. Similarly, Japanese Patent Application Laid-Open Nos. 2005-63586 and 2002-288830 disclose that at the beginning of writing a mark, a pulse with a power level lower than that of an erase pulse is inserted so as to prevent the top area of the mark from recrystallizing.
Japanese Patent Application Laid-Open No. 2001-273638 discloses that, when forming a mark with a 4T length or more, a bias pulse is inserted so as to prevent the mark from recrystallizing. The publication of Japanese Patent No. 2707774 discloses that, when forming a long mark by, for example, three write pulses or more, a bias pulse is inserted either before a top write pulse or after an end write pulse so as to sharpen the leading or trailing edge of the mark.
However, a further increase in storage capacity increases the recording density of the information recording layer, thereby degrading the quality of the reproduction signal. This makes it hard to identify a bit using zero-crossing detection. Moreover, this makes it hard to determine the signal quality using the jitter. Therefore, a Partial Response, Maximum Likelihood (PRML) detection scheme is needed for signal reproduction. Specifically, what is needed is a measurement of reproduction light in multiple levels to detect a reproduction response (reproduced waveform) and a choice of an ideal response suitable for the reproduction response. Accordingly, a distorted reproduced waveform causes an inadequate ideal response to be chosen, thereby leading to read error.
A study by the inventors, but is not in the public domain on the filing date of this application, showed that a recording state where a cycle of recording mark/space falls within the diameter of an effective spot for reproduction is subject to read error. They concluded that the fact that a cycle of recording mark/space falls within the diameter of the effective spot suggests that at least a part of another mark other than the recording mark is in the effective spot as well and that this another mark interferes in the recording mark so as to distort the reproduced waveform of the recording mark. In particular, a relatively short mark such as 2T, 3T, and 4T mark, which shows a reproduced waveform with a small amplitude, has an adverse effect on the choice of an ideal response even if the reproduced waveform is distorted a little.
All the patent documents cited above are to perform a precise pulse control of all marks or relatively long marks. Therefore, the application of such a control without any modification cannot solve the above problems and thus is unable to fully perform high-density recording.